where to deduct training costs required for employment

First you must ensure you have a signed T2200 Conditions of Employment and that the taxpayer earned commission income from that employer.

then you can claim on Line 20 T777WS …this will populate into the T777 Line 9270… which then populates on Line 229 of the T1 Jacket

IT-357R2… is worth a read to aid you in explaining to any non commissioned taxpayers why it is not deductible.